904 PROF. W. H. MILLER ON THE CONSTRUCTION OF THE NEW STANDARD POUND. 
Secondary Standards. 
The comparison of different brass weights with platinum weights, shows that, 
however carefully preserved, they are liable to gain from 0*01 grain to 0’02 grain or 
more in the course of a few years. Brass, therefore, unless very well protected by 
gilding, is quite unfit to be used in the construction of weights having that degree of 
accuracy which is required in secondary standards. Electro -gilding was tried in the 
first instance. It failed, however, to afford a sufficient protection to the metal 
underneath, and the weight of the pound when the gilding was completed, was so 
very uncertain as to render its adjustment an extremely troublesome process. It 
was afterwards discovered that these evils were due to the want of skill of the person 
employed as a gilder by Mr. Barrow, and not to any inherent defect in electro-gilding 
itself. Mr. Barrow then resorted to amalgam gilding. In order to adjust a weight, 
its under surface, which was slightly concave, was more thickly coated with gold than 
the rest of its surface. If too heavy, a little of the gold was removed by rubbing it 
with charcoal; if too light, more amalgam was added, and the mercury driven off by 
heat. Directions had been given that the weights should be made of the alloy used 
by Mr. Baily for the standard yard bars, consisting of thirty-two parts of copper, 
five of tin, and two of zinc. The densities, however, of the greater part of them 
indicate that these proportions have not been strictly observed. The lbs. numbered 
31 up to 36, protected by electro-gilding, were constructed by Messrs. Ladd and 
Streathfield. The weighings were reduced with the expansions given in Table III. 
The observations for finding the densities of the secondary standards, and for com- 
paring them with the platinum standard, were made in a room in the basement of 
my own house in Cambridge, the brick floor of which afforded a perfectly firm 
foundation for a strong table on which the balance was mounted. 
The weights employed in some of the weighings are of bronze, for which 
log A=0-92250. 
Apparent weights of the secondary standards in water. 
Water. 
Platinum. 
Bronze. 
Air. 
No. of lb. 
t. 
gr. 
gr- 
t. 
b. 
1 
15-11 
6163-2354 
17-03 
755-10 
2 
18-41 
6161-5534 
19-86 
760-32 
3 
17-75 
6157-7809 
19-94 
753-04 
4 
15-35 
6163-6422 
15-62 
767-02 
5 
14-69 
6132-0586 
16-45 
755-40 
6 
14-31 
6155-7369 
16-34 
758-92 
7 
14-24 
6138-4869 
15-17 
760-01 
8 
17-48 
6143-1594 
19-10 
760-86 
9 
16-31 
6047-7747 
17-01 
763-14 
10 
14-71 
6155-3347 
16-88 
758-12 
11 
17-80 
6163-6244 
18-49 
750-29 
12 
15-28 
6159-0015 
16-09 
767-20 
13 
10-79 
6169-9472 
12-20 
734-36 
14 
14-42 
6161-9845 
15-54 
755-94 
15 
10-23 
6162-9240 
11-01 
751-19 
16 
17‘43 
6133-5979 
17-93 
754-30 
17« 
11-15 
38-3615 
6099-9740 
12-42 
758-02 
